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I have had no luck in measuring speaker response at low frequency's,

Then you may not be using the right tools.

Almost any speaker system sold today claims 20Hz to 20Kz response yet this
is so far from the truth I cannot understand how they dare claim such
figures.

Because they (or you) are not providing all the information.

It isn't enough to say 20-20k, you must also include (for
the measurement to have Any validity) what the +/- dB range
is across the band. If you want a Truly accurate spec, it
will also include at what power level the measurement was
made, ie "maximum rated power" or "1 watt, 1 meter".

Most consumer and car speakers on the market fudge at least
one, if not all three of these qualifiers. Why? To take
gullible fools' money.

I have yet to find ANY speaker system using an 8 inch driver that has the
ability to produce 40hz or lower frequencys as they all have fallen so far
down in output level as to be useless.

See above. Also, there are significant physics issues
involved in trying to get an 8" driver to reproduce low
frequencies at any significant SPL. They simply can't move
enough air.

How do you measure the responese at 20Hz?, I have tested a few speakers and
basically I cannot get accurate (repeatable) data at much below 60Hz, even
using a borrowed shure KSM141 stereo pair in omnidirectional gives vague
results

First off, you should Never use a stereo mic (or any stereo
set up) for measurement purposes. You are guaranteed to get
inaccurate results. Secondly, the mic is only part of the
equipment. What else are you using?

I have come to the conclusion that you are
really measuring air displacement at anything under 30Hz

Ummm. You're measuring air displacement at Any frequency.
That's what audible sound IS.

and a flat panel
with a transducer is the only repeatable method of measuring the output,

Balderdash.

--
Mark

The truth as I perceive it to be.
Your perception may be different.

Triple Z is spam control.

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